This invention relates to facsimile systems comprising a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to a system wherein a document is scanned in a facsimile transmitter to generate electrical information-bearing signals representing the dark-light variations in the document being scanned. These information-bearing signals are then transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a reasonable facsimile of the original document.
Presently available facsimile transmitters and receivers are capable of different transmission modes. For example, the facsimile transmitters and receivers are capable of operating at different scanning rates depending upon the particular resolution required in a transmission so as to achieve different transmission times or durations. The QWIP 1000 facsimile transceiver manufactured by QWIP Systems Division of Exxon Enterprises is capable of such different scanning rates to achieve a 4-minute transmission mode as well as a 6-minute transmission mode.
Typically, the operator of a facsimile transmitter such as the QWIP 1000 is able to advise the operator of a facsimile receiver of the particular choice of a transmission mode. In other words, the facsimile transmitter operator can examine the document to be transmitted and determine the resolution required while also considering the increased cost involved in a long distance telephone transmission of 6 minutes versus 4 minutes. It is however desirable in some instances to provide for automatic signaling of the particular transmission mode so as to permit the receiver to automatically respond to and prepare for the transmission without intervention by an operator.
In order to provide for transmission mode signaling, the Model 410 transceiver manufactured by the Xerox Corporation frequency modulates the synchronizing pulse which is coincident with the juxtaposition between the photodetector and the clamp of a rotating drum on the transceiver. More particularly, a signal is transmitted from a transmitter having a frequency of 1500 Hz. during the synchronizing pulse for 6-minute transmission mode and a signal of 1250 Hz. is generated during the synchronizing pulse representing a 4-minute transmission mode. Since the Xerox 410 provides for a 15 second synchronizing period at the beginning of a transmission, it is possible for a receiver to detect the frequency of the synchronizing pulse and select the appropriate scanning rate before the document transmission begins. Typically, the duration of the synchronizing pulse is constant and the occurrence of the pulse relative to the rotation of the facsimile drum clamp past the detector is a function of the tolerance on a switch associated with the rotatable drum.
In addition to providing different scanning rate transmission modes, e.g. 4 or 6 minute transmission modes, it is desirable to provide for the selection of other transmission modes. For example, it is desirable to provide for the selection of different signals for synchronizing a facsimile transmitter and receiver. More particularly, it is possible to synchronize a facsimile transmitter and receiver on synchronizing signals which are internally generated by a crystal oscillator as shown in application Ser. No. 493,119 filed July 30, 1974. It is also possible to synchronize a facsimile transmitter and receiver on an externally generated signal such as the power line frequency. Heretofore, voice communication between the operator of the facsimile receiver and the operator of the facsimile transmitter has been utilized to assure that both the transmitter and the receiver were synchronized to the same synchronizing signal.